1. Field of the Invention
The present invention relates to a data conversion device, and particularly to a delta-modulation-type data conversion device.
2. Description of the Prior Art
For example, to achieve an echo effect or key control in a karaoke apparatus, or to achieve a surround-sound effect in an audio apparatus, it is necessary to add reverberations to an audio signal fed therein, or to store the audio signal in some way. In such cases, only real-time signal processing is possible as long as analog values are handled. For this reason, it is customary to convert the input audio signal into digital values first, then subject the digital values, as a digital signal, to signal processing and signal storage as mentioned above, then convert the digital signal back into an analog signal, and then output it as an audio signal.
A typical method for converting analog values to digital values is to use an analog-to-digital converter (hereafter referred to as an xe2x80x9cA/D converterxe2x80x9d) by which the time-varying voltage of an analog signal as sampled with predetermined timing is represented by a series of digital values each consisting of a plurality of bits in such a way that the actual voltage of the analog signal at one sampling point corresponds to the voltage represented by the digital value consisting of a plurality of bits taken at that sampling point. Another method is to use a delta-modulation-type signal conversion device (hereafter referred to as a xe2x80x9cDM-type signal conversion devicexe2x80x9d) by which an input signal to be converted is compared with a reference voltage that varies according to the previous comparison result to obtain a series of values representing voltage variations in such a way that the input signal is represented by a series of digital signals. Still another method is to use an adaptive-delta-modulation-type signal conversion device (hereafter referred to as an xe2x80x9cADM-type signal conversion devicexe2x80x9d) or the like that is developed to offer better trackablility than a DM-type conversion device.
By the use of an A/D converter, it is possible to directly monitor the voltage at a particular sampling point by referring to the corresponding digital value, and this makes digital signal processing easy. However, an A/D converter requires the use of extra components such as ladder resistors and comparators, and therefore needs to be incorporated in a semiconductor integrated circuit device that has a larger chip size and is more expensive. On the other hand, by the use of a DM-type or ADM-type signal conversion device, it is necessary to determine the voltage from a series of digital values, and therefore it is difficult to perform data conversion in such a way as to output a voltage on the basis of a digital signal at an arbitrary time point. However, a DM-type or ADM-type signal conversion device requires a far smaller chip size than an A/D converter, and therefore can be produced inexpensively. For these reasons, for digital signal processing in a low-cost audio apparatus, it is customary to use a DM-type or ADM-type signal conversion device.
A conventional delta-modulation-type data conversion device is disclosed, for example, in Japanese Laid-Open Patent Application No. H9-98089. In this delta-modulation-type data conversion device, to achieve efficient approximation of the input signal by the reference voltage, a current supplied from a current source is varied in steps, and this current is integrated to produce the reference voltage. An A/D converter incorporating this data conversion device converts an analog signal such as an audio signal into a series of digital signals and feeds them to a RAM (random-access memory) for temporary storage. Then, a D/A converter converts the digital signals stored in the RAM into an analog voltage by the use of a data conversion device having the same configuration as the above data conversion device, and then, by subjecting the analog signal to waveform shaping, specifically smoothing, achieved by the use of a low-pass filter, restores the analog signal.
Here, simply feeding the A/D-converted digital signals from the RAM to the signal-restoring-side data conversion device does not enable the signal-restoring-side data conversion device to determine the initial value of the absolute value of the current. As a result, as shown in FIG. 13, when the initial value of the absolute value of the current is great, the data stored in the RAM causes, in the D/A-converter-side data conversion device, the absolute value of the current to vary within a range in which it remains relatively great throughout, as indicated by the curve 30. By contrast, when the initial value of the absolute value of the current is small, the absolute value of the current varies in a range in which it remains relatively small throughout, as indicated by the curve 31, describing a waveform analogous to the curve 30. By integrating the current by the use of an integrating circuit in this data conversion device, a signal is output from the data conversion device. However, the amplitude of the signal voltage output from the D/A converter is different from that of the original signal.
To avoid this, when the A/D converter stores digital signals in the RAM, it is necessary to store, in addition to a series of digital signals, data of the initial value of the current so that the D/A converter first reads this data of the initial value of the current to set the initial value of the absolute value of the current and then restores the analog signal. Accordingly, when the D/A converter restores the analog signal, it is necessary to reset the D/A converter, set the initial value of the absolute value of the current in accordance with the data of the initial value, and then restore the analog signal, starting at the head of the digital signal. Thus, it is impossible to restore the analog signal starting at an arbitrary point other than the head of the digital signal.
An object of the present invention is to provide a delta-modulation-type data conversion device that does not require initial value data.
To achieve the above object, according to the present invention, a data conversion device for converting a series of digital signals into an analog signal is provided with: a current supplying circuit for supplying a current, the current supplying circuit being capable of varying the direction and magnitude of the current it outputs; an integrating circuit for integrating the current output from the current supplying circuit to output an analog voltage; and a control circuit for determining the direction of the current and whether to increase or decrease the magnitude of the current in accordance with the digital signals, the control circuit, according as the magnitude of the current increases, decreasing the value by which the magnitude of the current is increased and increasing the value by which the magnitude of the current is decreased.
According to this configuration, for example, if the analog signal output from the integrating circuit is compared with an analog signal fed from the outside by the use of a comparator, and the comparison result is fed to the control circuit, it is possible to realize a delta-modulation-type A/D converter that outputs the digital signals output from that comparator. When the thus obtained data is subjected to D/A conversion by the use of the data conversion device of the invention, if the absolute value of the current in the control circuit is great, the data conversion circuit decreases the value by which the absolute value of the current is increased and increases the value by which the absolute value of the current is decreased; by contrast, if the absolute value of the current is small, the data conversion circuit decreases the value by which the absolute value of the current is increased and increases the value by which the absolute value of the current is decreased. Thus, when the initial value of the absolute value of the current in the D/A converter is smaller than the initial value of the absolute value of the current on the A/D conversion side, as time passes, the absolute value of the current gradually increases, and, when the initial value is greater, the absolute value of the current gradually decreases, eventually converging on the original data. As a result, there is no need to previously store the data of the initial value of the current in the A/D and D/A converters in order to restore the original data starting at an arbitrary point within a series of digital data.